1. Field of the Invention
The present invention generally relates to an optical pickup. More specifically, the present invention relates to an optical pickup having a front monitor that detects luminous energy of a light beam directed at an optical recording medium.
2. Background Information
An optical disk device is an information recording and reproduction device. The optical disk device records and reproduces to and from low-cost media with high-speed access, large capacity, and non-contact operation. The optical disk device has been used as recording and reproduction devices for digital video and audio, and as external memory devices for computers. There are many different kinds of optical disk, with varying information recording capacities or wavelength compatibilities. For example, the recording capacity is about 650 MB when a laser beam in the 780 nm band is used on a 12-cm optical disk such as a CD-ROM or a CD-R. The recording capacity is about 4.7 GB when a laser beam in the 650 nm band is used on a DVD-ROM, a DVD-R, or the like.
CD-R and DVD-R disks have a large recording capacity and are inexpensive media, which is why their popularity has soared in recent years. When recording to a CD-R or DVD-R disk, the recording has to be carried out under the optimal recording power condition for a particular optical disk. Accordingly, an optical pickup installed in an optical disk device that records information to the optical disks includes a front monitor to perform recording at the optimal recording power. The front monitor accurately monitors emission luminous energy of an objective lens by receiving part of a light beam in front of a semiconductor laser.
Specifically, the luminous energy of the light beam emitted by the semiconductor laser is monitored, and the luminous energy of the light beam is kept at a constant value. With a front monitor system, part of the light beam is separated and the luminous energy is monitored. A beam splitter is disposed along an optical path of the light beam, and this allows part of the light beam to be separated at a constant ratio.
Furthermore, with the front monitor system, the luminous energy of the separated light beam (hereinafter referred to as monitor light) is detected by a light receiving element. Thus, with the front monitor system, the luminous energy of the received monitor light is controlled to be a specific value. As a result, the recording and reproduction luminous energy emitted from the objective lens is accurately controlled to a specified luminous energy. This is called a front monitor type of auto power control (hereinafter referred to as APC).
Recording speed need to be raised to reduce recording time for the CD-R and DVD-R disks with high-capacity. To raise the recording speed, it is necessary to rotate the optical disk at a higher speed and, at the same time, take less time to converge the light beam and form recording pits necessary to record on the optical disk, which means that emission power of the semiconductor laser used for recording has to be boosted. Consequently, an output of the semiconductor lasers installed in optical pickups has been increased every year.
A property of high-output semiconductor lasers is that an intensity distribution of the laser varies when laser emission power is changed. If the intensity distribution of the laser varies, then the intensity distribution of the light beam incident on the objective lens will vary. As a result, the power of the light beam emitted from the objective lens and converged on the optical disk will also vary. Similarly, with the front monitor, if the intensity distribution of the laser varies, then the intensity distribution of the light beam incident on the front monitor will also vary. As a result, the luminous energy detected by the front monitor varies. Accordingly, if the emission power of the laser varies, then the ratio of the luminous energy emitted by the objective lens to the luminous energy detected by the front monitor will vary. As a result, an error in the luminous energy emitted from the objective lens as estimated from the luminous energy detected by the front monitor occurs. The error in the luminous energy emitted from the objective lens adversely affects recording performance because the recording operation cannot be carried out at the optimal power level for the optical disk.
Also, when the emission power of the laser is changed, there is a tendency for a diffusion angle of the light beam to spread out if the emission power is raised. Japanese Laid-Open Patent Application Publication No. 2003-317296 discusses a technique for keeping luminous energy of a light beam constant with respect to changes in an intensity distribution of the light beam caused by spreading of a diffusion angle that accompanies a change in emission power.
The Japanese Laid-Open Patent Application Publication No. 2003-317296 shows a positional relationship between a front monitor and an optical axis of a light beam. Furthermore, the Japanese Laid-Open Patent Application Publication No. 2003-317296 shows a layout of the front monitor that receives the light beam emitted from a semiconductor laser, and a spread angle of the light beam. A light detection face of the front monitor is generally disposed in a center with respect to the optical axis of the light beam. A surface area of a light receiver of the light detection face is limited by frequency characteristics of the optical pickup. Not all of the light beam emitted from the semiconductor laser can be received, but part of the light beam is always received.
The center of the light detection face of the front monitor is disposed at a location a certain distance away from the optical axis of the light beam emitted from the semiconductor laser. Therefore, with the light detection face of the front monitor, the light beam is detected at a point where the amount of change in the light beam intensity is large, such as a point near the midsection of the intensity distribution of the light beam emitted from the semiconductor laser. The effect of thus disposing the light detection face of the front monitor is that the light beam intensity resulting from a change in the spread angle can be detected at the light detection face with a limited light receiving surface area. Furthermore, the luminous energy of the light beam can be kept constant so that the ratio of the luminous energy detected by the front monitor to the luminous energy emitted by the objective lens is substantially constant.
With the technique discussed in Japanese Laid-Open Patent Application Publication No. 2003-317296, it is possible to keep the luminous energy of the light beam constant so that the ratio of the luminous energy detected by the front monitor to the luminous energy emitted by the objective lens is substantially constant with respect to changes to the spread angle related to the emission power of the semiconductor laser. However, no thought is given for offsetting of an optical axis angle of the light beam caused by changes in temperature of the semiconductor laser. Thus, the luminous energy of the light beam cannot be kept constant with respect to offset to the optical axis angle. As a result, the APC does not operate properly.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved optical pickup. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.